Tone control system



Oct. 26, 1937.

E. S. PURINGTON TONE CONTROL SYSTEM 3 Sheets-Sheet l dim.. 1:6 0302 4 OJl 'u 'l mozruarulh Filed April 3, 1936 n w/ M, F

V & WH/.M w w .mo m .m w n Em Oct. 26, 1937; E. s. PURINGTON TONECONTROL SYSTEM I Filed April 3, 1936 I: Sheets-Sheet 2 o dIQDT LINVENTOR Ellison S. Pur'ingfon. BY

v ATTO NEY Oct. 26, 1937.

E.- S. PURINGTON TONE coNT RoL SYSTEM Filed April 3, 1956 K i l 0 cy c las 3 Sheets-Sheet 3 KHocycles .l'as .25

' INVENTOR EH ison S Puvi n gTOh RNEY' Patented on. a e, i937 on rreosr2,096,760 TONE ooN'raoL srs'rsM Ellison S. Purington, Gloucester, Mass,assignor to John Hays Hammond, Jr., Gloucester, Mass.

Application April 3, 1936, Serial No. 72,520

21 Claims. (Cl. 179-1001) This invention relates to the reproduction ofsound from a sound record and has for its purpose to provide automaticmeans for controlling the tonal characteristics of the reproduced sound.

More specifically the invention provides an automatic means fordiscriminating against the reproduction of sound of a given frequencyrange, unless there are suitable signals in that frequency range.

The invention further .provides for the masking of background noises, bysuppressing the transmission of noise frequency signals unless thedesired signals are of suitable nature to mask the presence of thenoises. The invention also provides for increasing the tonal contrastsof the reproduced sound.

In accordance with the present invention, the tone control is effectedby providing a transmission channel having a variable filter networkwhich includes the primary of a transformer,

the secondary of which is variably loaded by a thermionic impedance. Thethermionic loading is controlled by voltages obtained by rectifyingsuitable energy derived from the tonal energy output of the soundrecord.

' The control action depends upon the principle that the effectiveprimary impedance of a transformer, which inthis case determines theimpedance properties as far as the transmission network is concerned,.is dependent upon and controllable by the impedance loading of thesecondary circuit. The general effect of lower;

ing the value of a resistance impedance shunted across thesecondary-winding of a transformer, is to increase the secondary loadingand to reduce the equivalent primary impedance, especially for thehigher frequencies. The relative transmission of currents of differentfrequencies through the network in which the primary of the transformeris connected is therefore modified by secondary thermionic impedancecontrol.

A feature of the invention accordingly consists in replacing aninductance element of a conventional filter, such as a low pass, bandpass or high pass filter, by the primary of a transformer having avariable secondary thermionic loading, whereby the properties of thefilter may be controlled, as to cut-off frequency,'sharpness of cut-offetc., inaccordance with the energy by which the thermionic impedance ismodified. 'Another feature, consists in connecting the secondary to apair of thermionic impedances, each carrying direct current, with abalanced secondary winding arrangement such that the as equivalentprimary impedance is controlled solely by the alternating currentthermionic impedance values, and is not modified by the changes ofdirect current through those impedances. A wide range of transmissionvalues is thus achievedby one transformer, the balanced operationminimizing any distortions that may result from non-linearity of tubecharacteristics. Furthermore a rapid tonal control is obtained due tothe fact that no change of the direct current flux of the core occursduring operation.

The invention also consists in the various new and original features ofconstruction and combinations of parts hereinafter set forth andclaimed.

Although the novel features which are be lieved to be characteristic ofthis invention will be particularly pointed out inthe claims. ap--pended hereto, the invention itself, as to its objects and advantages,the mode'of itsoperation and the manner of its organization may bebetter understood by referring to the'following de' scription taken inconnection with the accompanying drawings forming a part thereof, infwhich Fig. 1 illustrates diagrammatically the system as applied to thereproduction of sound from the sound record of a talkingmoving pictureprojector and phonograph.

Fig. 2 is a series of curves illustrating the relative frequencytransmission characteristics of certain portions of the circuit shown inFig.1.

Fig. 3 is a schematic diagram of a modified form of the filter circuitshown in Fig. 1;

Fig. 4 is a series of curves illustrating the relation between thefrequency and the transmission characteristics of the modified circuitshown in Fig. 3. V

Fig. 5 shows a circuit to illustrate the insertion loss which may beproduced in accordance with this invention. 40

Fig. 6 is a series of curves illustrating the ing sertion losses of thecircuit depicted in Fig. 5.

Fig. 7 shows a circuit to illustrate the insertion loss which may beproduced with a different circuit arrangement. 45

Fig. 8 is a series of curves illustrating the insertion losses of thecircuit depicted in Fig. '7.

Like reference characters denote like parts in the several figures ofthe drawings.

In the following description and in the claims 50 parts will beidentified by specific names for convenience, but they are intended tobe as generic in their application .to' similar parts as the art willpermit.

Referring to the accompanying drawings, and

I more than 25 to50 percent as the space current more particularly toFig. 1, a phonograph pick-up In is shown which is connected to twocontacts of a double-pole, double-throw, transfer switch II, the othercontacts of which are connected to the circuit of a photo-electric celll3 including bat tery l5. Light from a lamp I5 is focused by means of alens l1 through an aperture la in an aperture plate l9 upon the soundrecord of a moving photographic film 23. The light which passes throughthis film then impinges upon the photoelectric cell l3.

The blades of the transfer switch II are connected to the primarywinding of input transformer 2|, one side of the secondary of which isconnected to ground and the other to the grid of a separate heater typetriode amplifier tube 22.

The cathode of this tube is connected to ground through bias resistor33, in parallel with by-pass condenser 3|. The plate supply for tube 22is taken from battery 23, the negative side of which is grounded, andthe positive side of which is connected through feed resistor 28a andcoupling resistor 29 to the plate of tube 22. A by-pass condenser 23b isconnected from the junctions of feed resistor 28a. and coupling resistor29 to ground. Battery 28 serves also as plate supply to other tubes tobe mentioned later.

A blocking condenser 23, the primary of the tone control transformer 25and one resistance winding 25 of a manual volume control rheostat areconnected in series between the plate of the amplifier tube 22 andground. The ends of the secondary windings of transformer '25 areconnected to the plates of two space discharge tubes 32, the alternatingcurrentimpedances of which control the tonal transmission in a manner tobe described. Space current for the tubes 32 fiows through feed resistor35, one terminal of which is able contacts to resistance winding 21, oneend of 5 which is connected to primary of coupling transformer 31, ofwhich the other end is connected to connected to the positive side ofbattery 28* and v the other terminal to the center tap of secondary oftransformer 25. The secondary winding is so arranged that the D. C.space currents'to the two plates produce opposite magnetizing effectsupon the core of the transformer. The cathodes of tubes 32 are joinedtogether, and to the junction of bleeder resistors 33 and 34, which inturn are connected in series between center tap of the secondary oftransformer 25 and ground. Resistor 34 serves also as a cathode biasresistor, and the circuit is so designed that the value of the voltageacross this resistor 34 does not increase of tubes 32 changes from zeroto maximum value. The grids of tubes 32 are connected together andbetween them and ground are two resistors 54 and 53, the functions ofwhich will be described later. The cathodes of tubes 32 are connected toone side of filtering and timing condenser 35, the other side ot whichis connected to the grids. The impedances of tubes 32 are controlled bythe voltage across the condenser 35, which varies in accordance with thenature of the signal output of amplifier tube 22 in the manner to bedescribed.

The winding of the transformer 25 which is connected to the plates oftubes 32 is referred to as a secondary winding because the tubes 32serve as power consuming devices, and not as an amplifier of tonalfrequencies. The balanced nature of the windings prevents variations ofvoltage' across grid-cathode condenser 35 from producing similarvariations, of current through the primary winding of the transformer.The impedance loading upon the secondary of transformer ground. Thesecondary terminals of transformer v 31 are connected in a be ancedfashion to the grids of two driver amplifier triode tubes 38 of 10 theseparate heater type, and the center tap of the secondary of thetransformer is connected to ground. The cathodes of tubes 38 areconnected together and connected to one side of bias resistor 45, theother side of which is connected to ground. In parallel with resistor 45is a bias by-pass condenser 46. The plates of tubes 33 are connected tothe primary winding of coupling transformer 39, the center tap of whichis connected to the positive side of battery 23. The 39 secondarywinding of transformer is connected in a balanced, fashion to the gridsof two power amplifier triode tubes 40, and the center tap of thesecondary winding is connected to the' negative side of bias battery 41,the which is connected to ground. The filaments of tubes 40 areconnected in parallel, and are bridged by center tapped filamentresistor 48, the center tap being connected to ground. The

plates of tubes 40 are connected to the primary 30 of output transformer4|. The center tap of the primary of the transformer 4| is connected tothe positive side of power supply battery 44, the negative side of whichis grounded. The secondary of transformer 4| is connected to the voicecoil 35 winding of loud speaker 42. The field winding 43 of loudspeaker42 is connected across battery 44.

For producing variation in the control voltage across condenser 35, someof the energy amplified 40 by tube 22 and fed through condenser '23 isdiverted to the grid of amplifier tube 5| of the separate heater type.For this purpose, a filter coupling network 49 is connected between thejunction of condenser 23 and primary of transformer 25, the grid of tube5|, and ground, this filter being of the high pass type and terminatingin resistor 53 connected between grid of tube 5| and ground. Thisresistor 50 serves to terminate the filter, and to make the operating or50 average grid potential substantially that of ground. The cathode ofthe amplifier tube is connected to one end of bias resistor 52 the otherendof which is connected toground. In parallel with resistor 52 isby-pass condenser 53. Plate-m current for tube 5| is supplied from thepositive side of battery 28, and fiows to the plate through feedresistor 58, and coupling resistor 55. The

connection between resistors 55 and 53 is connected to one terminal ofby-pass condenser 51, 00

the other terminal of which is connected to ground. The plate ofamplifier tube 5| is connected to one terminal of the blocking andfiltering condenser 55a, the other terminal of which is connected to oneterminal of poten- (E3 tiome'ter 56, the other terminal of which isconnected to ground. The variable contact of the potentiometer 55 isconnected to the grid of triode amplifier tube 54 of the separate heaterminal of the primary, of coupling transformer 75 positive side of 5:5

The cathode of tube 54 is connected to 70 6| the other terminal of whichis connected to an end of feed resistor 65, the other end of which isconnected to the positive side of battery 28. The Junction of resistor65 and primary of transformer 6| is connected to one terminal of by-passcondenser 6511, the other terminal of which is connected to ground. Oneterminal of the secondary of transformer 6| is connected to the cathodeof the triode type rectifier tube 62 of the.

separate heater type. The other terminal of the secondary is connectedto one end of rectifier load resistor 63 and also to one end ofresistor- 64 which serves as a coupling, timing and limiting resistor.The other end of resistor 63 is connected to ground, and the other endof resistor 64 is connected to the grids of tubes 32. The

up ill or the photo-electric cell 3 passes to the amplifier 22, where itis amplified and fed through the blocking condenser 23 and primary ofthe tone control transformer 25 to the volume control adjustablerheostat windings 25 and 21. From the winding 21 theenergy passesthrough the transformer 31 to the input of the driver amplifier tubes38. Here it is further amplified and then passes through the transformer39 to the input of power amplifier tubes 40. The output of the poweramplifier tubes 40 then passes through the transformer II to the loudspeaker 42, where it is reproduced as speech or music in the usualmanner.

For controlling the operation of the automatic variable filter, some ofthe energy from the output circuit of the amplifier 22 passes throughthe high pass filter network 49 to the amplifier 5|. Here the energy isamplified and fed through the potentiometer 56 to the second amplifier54. Here the high frequency energy is further amplified and is fedthrough the transformer 6| to the rectifier 62, where it is rectified toproduce a current fiowing unidirectionally through resistor 63 in thedirection of the arrow.

' The pulsations of rectified current are smoothed by resistor 64 andcondenser 35 to produce a control potential on the grids of tubes 32.The amount of potential change produced is dependent upon the strengthand frequency distribution of electrical energy in the output ofamplifier tube 22, and the grids become more positive as the energy isincreased without change of distribution, or as the energy distributionis shifted toward the high frequencies without change of energy. In theabsence of signals, the tubes 32 are biased sufliciently by resistor 34to reduce the space currents substantially to zero so that the secondaryof transformer 25 is not loaded, and the effect of the presence of thetransformer primary is principally the choking effect due to the actualinductance of the primary winding. This discriminates against thepassage of higher frequency signals, including surface noise, forexample in the range of 1000 to 4000 cycles.

The discrimination against the higher tones is shown by curve III inFig. 2 which shows the lowered transmission of the higher frequenciesfrom tube 22 to the volume control resistance 25 caused by the filterincluding the primary of transformer 25, when the rectifier currentthrough resistor 83 is small and the control tube impedances are high.This corresponds to the transmission when the needle is in the recordgroove in the absence of any appreciable recorded signal capable ofcausing rectification by the tube 62. 1

When there are recorded signals of sumcient strength or of suificientlyhigh tone to mask the presence of the background noise discriminatedagainst by the choking action, the rectifier circuit is operated toreduce the bias on tubes 32. This decrease is in accordance with thecharacteristics of the control channel as for example shown by curve 1|,which shows the relative transbe produced by a small amount of highfrequency energy or. a larger amount of lower frequency energy. When therectified energy is large so that the impedances of the control tubes 32are low, and the equivalent primary inductance very small, thetransmission of the main channel is according to curve 12, with greatlyimproved transmission for the higher frequencies over that shown bycurve 10. While the condition illustrated in curve 12 permitsthe passageof background noises, this occurs only when desirable signalssufficiently strong in the background noise range, or much stronger inthe lower tonal ranges, exist; hence the presence of the backgroundnoises will be completely masked. The constants of the rectifier-circuitare so chosen, as by making capacity of condenser 35 small, that controlaction is rapid and changes of transmission characteristics occur changeof the nature of the signals.

This speed of action is not hindered by any delay of the plate circuitoperation, since the change of plate current does not change themagnetic energy content of the iron core, and therefore no inductivevoltages are produced across either half of the secondary windings oftransformer 25.

In conventional amplifiers, fixed scratch or noise filters are used,with condensers and inimmediately upon ductors which are adjusted foroptimum transmission under conditions of weak or no signals,corresponding for example to curve 10 of Fig. 2. By this invention, witha rapidly acting automatic tone control system, the freqeuncy range ofany type of record may be effectively increased, since the filteringaction applied against tones in the higher frequency range isautomatically reduced so that the filtering action is never greater thannecessary for a given signal energy distribution. The circuit therefore,for a given type record, operates to reproduce on the average muchhigher frequency tones than would be permissible if background orsurface noises were suppressed by-co-nventional type scratch filters.

In the modified form. of the invention shown in Fig. 3 the phonographand photo-electric pickwith those of Fig. 1 and also have been omitted.In Fig. 3 the secondary terminals of the input transformer 2! areconnected to the grids of two space discharge tubes BI and 82 of apushpuil amplifier 30. These tubes are of the separate .heater type. Thecathodes are connected together and to one terminal of bias-resistor aand of bias resistor by-pass condenser 80b, the other terminals of whichare grounded.

Plate supply for the plates of tubes 8| and 82 is taken from battery H8,the negative side of which is grounded. This battery serves as supplyalso for other tubes of Fig. 3. Filaments of tubes 8i and 82 are heatedby a source not shown, preferably a secondary winding of a transformer,the center tap of which is grounded or connected to the cathodes of thetubes which are heated thereby. It will be understood that the filamentand plate supply may be from any suitable source. Bridged across from.plate of tube 6! to plate of tube 82 is a variable low frequencyabsorption circuit and plate feed circuit 38 comprising two resistors89, two chokes 9t and the primary of a tone control transformer 92,across whichis shunted a tuning condenser The center tap of the primaryof transformer $52 is connected to the positive terminal or" battery M8.

Also bridged from plate of tube iii to plate oi tube it? is a variablehigh frequency power ab sorption circuit and intertube coupling circuitcomprising in series one primary of coupling transformer 83, the primaryof the tone control transformer 9'? and the other primary of couplingtransformer 38.

The secondary terminals of tone control transformers t2 and ii? areconnected in a balanced fashion to the plates of two sets of tonecontrol space discharge triode tubes 35 and 328 of the separate heatertype. Center taps of sec ondaries of these transformers are connected toone terminal or a feed resistor lit, other ter minal of which isconnected to the positive terminal of the battery li'ii. Between thecenter tap of the'secondary of transformer and ground are connected inseries the bleeder resistor iZ l and the tapped cathode bias and limiterresistor i ll. Similarly connected between the center tap of thesecondary of transformer and ground are bleeder resistor M5 and cathodebias re sistor M6. The cathodes of the tubes Q5 are joined together andto the junction of resistors i2 5 and ill, and the cathodes of the tubes98 are joined together and to the junction of resisters E25 and 528. Thegrids of tubes 95 and. 98 are connected together, and between them andground are connected in series, the resistors H6, H5, and M9, thefunctions of which will be described later. Filtering, timing andbypass-= ing condensers 95a, 98a, 95b, and 932) are connected betweenthe grids and cathodes of tubes 95 and 98, respectively, and between thecathodes of tubes 95 and 98 respectively and ground.

The secondary terminals of the coupling transformer 83 are connected totwo terminals of manual volume control potentiometers 85 and 86, theother ends of which are connected together, to ground and to the centertap of the secondary of transformer 83. The adjustable contacts ofpotentiometers 85 and 86 are connected to the grids of a pair of tubes81a and 81b of a push-pull driver amplifier 81. These tubes are of theseparate heater type. The cathodes are connected together, and betweenthem and ground are connected in parallel a. cathode bias resistor 81cand a cathode bias resistor by-pa s's condenser 81d.

\ The plates of tubes 81a and 81b are'connected to the primary terminalsof transformer 39, the center tap of which is connected to the positiveend of battery 8.

As shown in Fig. ;1, and explained above, the secondary windings 39 areconnected to the grids of a power amplifier 40 which in turn operates a.loud speaker 42.

For controlling the potentials of the grids of tubes and 98 there isprovided between plate of tube 82 and grid of a triode amplifier tube N3of the separate heater type, and ground, a high pass filter ill! of thecondenser-resistor type with condenser elements in series and resistorelements in parallel. Connected between grid of tube 503 and ground is afilter-terminating and bias-determining resistor I02. Between theoathode of tube ")3 and ground are in parallel a cathode bias resistorI030 and by-pass condenser with.

Between the positive terminal of battery H8 and plate of the tube 33 areconnected in series 2. feed resistor iii ands. coupling resistor W4.Between the junction of the resistors i2l and lil i and ground isconnected a by-pass condenser iilt Etween the plate oi tube I63 andground is connected an output circuit comprising blocking condenser iotaand potentiometer we in serice, the variable contact of potentiometerbeing connected to the grid of the triode ampliiier tube till. Betweenthe cathode of the tube to? and ground are connected in paraliel a biasresistor and by-pass condenser sou). Between the positive endof thebattery l lii and the plate of tube 5%? are connected in series a feedresistor 52?. and. a coupling resistor 66%, the junction of whichconnected to one terminal of bypass condenser M2, the other terminal ofwhich is grounded.

Between the ground and plate of tube llil is connected an amplifieroutput circuit including a first plate-cathode branch of a double unidirectionally conductive tube H3, in parallel with a rectifier circuitresistor ms. A blocking condenser ill is connected from the junction ofresistor hi9 and the first cathode of tum H3. From this junction to thesecond plate of tube H2; is a smoothing and limiting resistor H5, andfrom the second plate of tube H3 to the grids of tubes and lit is asmoothing and limiting rcsistor i it. The cathode of the secondplate-cathode branch of tube i i3 is connected to the tap on resistorill. I

It will be understood that while condensers soc, H2,-lfl3b, Niki-and Illhave been described as by-pass and blocking condensers, the values ofcapacitance may be chosen less than if the amplifier circuits of tubesI03 and I01 were desired to amplify uniformly at all frequencies.Lowering the capacitance of any of these condensers decreases theability of the amplifier circuits to amplify low frequency tones.Accordingly the entire filtering action from plate of tube 82 to thefirst cathode-plate branch of tube I I3 is not concentrated in filter H,but involves further discriminatory action of the amplifier circuits infavor of the higher tones.

In the operation of the modified form of the invention shown in Fig. 3,the energy from the pick-up device passes through the transformer 2| tothe push-pull amplifier 80, where it is amplified and then passesthrough the transformer 83 to the second push-pull amplifier 81. Here itis further amplified and passes through the transformer 3% to the poweramplifier 40 shown in Fig. 1,

" thence through the transformer 4| to'the loudspeaker 42 where it isreproduced as speech or music in the usual manner, The transmission fromtubes 8| and 82 to tubes are and 81b is dependent upon the internalimpedances of the tone control tubes 95 and 98, which in turn aredependent upon the potential of the grids of these tubes with respect tocathodes. In general lowered impedances of tubes 98 decrease the chokingaction of the primary winding of transformer 83 in a manner similar tothat of similar elements of Fig. 1. This results in increasedtransmission of higher frequencies. On the other hand, the circuit 88shunted from plate of tube 8| to plate of tube 82 is designed so thatchokes 98 prevent the absorption of higher frequency energy, and thiscircuit modifies the transmission chiefly for low frequencies. Thelowering of the internal impedances of tubes 95 results in a loweredpri-- mary impedance of transformer 92, and more loss oftransmissionforthe low frequencies.

For producing control voltage, some of the energy from the output oftube 82 is fed through filter IM to the grid of amplifier tube I83. Thefilter elements of I8I are of high impedance and the amount of energydiverted to amplifier I83 is relatively small. The filtered energy isselectively amplified by tube I83 and associated circuits, and impressedupon the grid of tube I8'I. It is again selectively amplified andimpressed upon the first plate-cathode branch ,of the tube II3. Thistube rectifies due to its undirectional characteristics, since it passescurrent only when the cathode is negative with respect to the groundedplate. The rectifier resistor I89 carries A. C. tonal current and alsorectified D. C. current, the direction of which is indicated by thearrow. As a result the D. 0. potential of the first cathode becomespositive with respect to ground although the potential goes negativeinstantaneously when the rectifier is operating. The pulsations ofpotential of this cathode are smoothed out by resistors H and H6 inseries, and condensers 95a, 95b, 98a, and 981 so that a potentialbetween grids and cathodes of the tubes 95 and .98 exists substantiallydevoid of the high tonal frequencies present in resistor I89. Thispotential changes with change of the amount of energy impressed upon therectifier circuit. The second plate-cathode branch serves as a spillwayfor rectified energy whenever the potential of the plate exceeds thepotential of the cathode which is the same as that of the tap onresistor IN. This makes the potential of the grids of tubes 95 and'98change more on a proportionate basis when the tonal signal energy is lowthan when it is high. It is understood, however, that even if thelimiting action of the second plate-cathode branch is not used, limitingactions will occur when the grids of tubes 95 and 98 become positivewith respect to the cathodes, due to the resultant voltage drop throughresistors H5 and 5 against low tones when -high tones are especiallypresent.

' The transmission cha acteristics may be better understood withreference to Fig. 4, in whichtransmission of various parts of the systemis shown as dependent upon the frequency of the energy beingtransmitted. Curve I28 illustrates thetransmission of energy through thecontrol channel, showing the relative ability of energies of differentfrequencies at the secondary of transformer 2| to produce controlpotential on the grids of tubes 95 and 98. Curve I2I shows the relativetransmission of energies of different fre-' quencies from thetransformer 2| to transformer 39 under the condition of the controlcircuit passing negligible energy, and curve I22 shows the same underthe condition of passage of maximum potential change due totransmissionof' a sufflciently large amount of energy through thecontrol circuit.

It will be understood that musical selections utilize volume contrastsand tonal contrasts. In addition to the use of the circuits of .Fig. 1and Fig. 3 to minimize surface noise effects, they may be used toemphasize the tonal contrasts. Thus at a given acoustical volume level,when the emphasis is intended to be upon the low tones, the circuitsautomatically increase the emphasis on the low tone, and when theemphasis. is intended to be upon the high tones, the circuitsautomatically increasethe emphasis on the high tones.

It will be understood, however, that the use of a transformer with anautomatically variable loading is not limited to the arrangementsspecifbe achieved by use of certain transformer arrangements incombination with condensers and resistors. The curves of Fig. 6 show theinsertion loss produced by the circuit of Fig. 5, when transformer I38and primary condenser I3I, with different secondary loadings due toresistor I31 and thermionic impedance in parallel, are cut in betweenresistors I32 and I33. The insertion loss is measured by the decibeldecrease of energy to resistor I33 when switch I35 is transferredicallyshown. For example, in Fig. 5 to Fig. 8 are showntypical insertionloss effects which can from the up position to the down position. Sim

ilarly the curves of. Fig. 8 show the insertion loss producible by thearrangement of Fig. '7, for

which the network consists of a transformer I42- with variable secondaryloading, and with a condenser I48 and resistor MI shunting the primary,the transfer switch connecting the transformer primary in parallel withthe receiving resistor.

While in the circuit of Fig. 5 the variable impedance element especiallycontrols'the passage of high frequency tones, in the circuit of Fig. 6it especially controls in an inverse sense the passage of low frequencytones.

- This control may be in accordance with any desired tonal stimulus,with increased signal causing either increased or decreased loading. Itwill be understood that the invention is not limited to the arrangementof Fig. 1 or Fig. 3. For example it is within the scope of the inventionto use the control arrangements of Fig. '7

and Fig. 8, in an automatic tonal compression" device, for use inproducing records. For this purpose the low secondary impedanceR.=10,000 ohms would correspond to strong signals, and the amount ofvolume compression would be especially great for the lowest tones.Increased effects may be produced by using a plurality of tone controlelements preferably in different stages of the 6 phonograph or talkingpicture recorder or reproducing amplifier.

' While certain specific embodiments of the lnvention have been setforth for purposes of 11 lustration, it is to be understood that variouschanges and modifications may be made therein as will appear to a personskilled in the art. The scope of the invention is only to be limited inaccordance with the following claims when interpreted in view of thepriorart.

What is claimed is:

1. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein including a choke coil, saidchoke coil constituting a primary of a transformer, means applying abalanced load to the secondary of said transformer whereby variations insaid load are prevented from reacting on the flux in the core of saidtransformer and means responsive to the signal energy for automaticallyvarying said load so as to change the characteristics of said filter inaccordance with the characteristics of said signal energy.

2. In a system for recording or reproducing sound, a transmissionchannel havinga filter network therein including a choke coil, sai chokecoil constituting a primary of a transformer, a pair oi variableimpedance devices connected across the secondary of said transformer soas to provide balanced, load thereon, whereby the variations in load areprevented from reacting on the'flnx in the transformer core and meansresponsive to the signal energy for automatically varying the impedanceoi said devices soas to change the load on said. transformer inaccordance with the characteristics of the signal energy.

S. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a transformer having a primaryand a balanced secondary, said primary being disposed in said filternetwork, means applying a variable load to said secondary, said loadbeing adapted to vary the effective primary impedance withoutsubstantially changing the flux density of the'core, the primaryimpedance when the secondary is unloaded serving to prevent passage ofthe high frequency components of the signal and, when the secondary isloaded, serving to progressively pass more of said high frequencycomponents, and means responsive to the certain frequency components of.the signal for controlling said load.

4. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a transformer having a primaryand a balanced secondary, said primary being connected in said filternetwork so as to control the transmission characteristics thereof inaccordance with the primary impedance, a pair of space discharge devicesconnected to the balanced secondary and adapted to variably load thesame for altering the effective primary impedance of said transformer, acontrol channel responsive to certain components of the signal,

means in said control channel to produce a control voltage proportionalto the strength of said components and means applying said controlvoltage to said space discharge device so as to vary the impedancethereof in accordance with the strength of said components.

5. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a choke coil in said network,means to vary the impedance of said choke coil without changing the fluxdensity thereof and means responsive to the signal energy forcontrolling said last means so as to vary the transmissioncharacteristics oiLthe filter in accordance with applied signal.

6. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, an inductive impedance in saidnetwork, means varying the value of said impedance in accordance withthe applied signal and means to prevent such changes in impedance fromcausing a change in the direct current flux of said impedance wherebythe timing is not infiuenced by the reluctance of the flux path.

7. In a system for recording or reproducim sound from a, sound record, atransmission channel having a filter network therein including aninductive impedance device, and means applying a variable balanced loadto said device which.

and a balanced secondary, meansapplying a var-=- iable load to saidsecondary, means responsive to the signal energy for automaticallyvarying said lead in proportion to the change in signal energy, saidprimary being so disposed in said filter network that the impedancethereof, when the secondary is unloaded, prevents passageoi highfrequency components and an increase in the secondary load serves tolower the primary impedance so as to permit passage of the highfrequency components.

9. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a transiormer having aprimaryand a balanced secondary, said primary being disposed in said filternetwork, means epplying a variable load to said secondary, said loadbeing adapted to vary the eilfective primary impedance withoutsubstantially changing the fins: density of the core, the primaryimpedance, when the secondary is unloaded, serving to prevent passageoi. the high. frequency components of the signal and, when the secondaryis loaded,

serving to progressively pass more of said high frequency components,and means responsive to the high frequency components of said signal forvarying said load in proportion to the strength and frequencydistribution of said components,

wherebysaid components are passed by said filter network only when theyare of a predetermined minimum strength.

10. In a system for recording or reproducing sound, aftransmissionchannel having a filter. network therein, a transformer having a primaryand a balanced secondary, said primary being connected in said filternetwork so as to control the transmission characteristics thereof inaccordance with the primary impedance, 9. pair of space dischargedevices connected to the balanced secondary and adapted to variably loadthe same for altering the eflective primary impedance of saidtransformer, a control channel responsive to the high frequencycomponents of the signal, means in said control channel to produce acontrol voltage proportional to the strength of said space dischargedevices so as to vary the impedance thereof in accordance with thestrength of said components.

11. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a transformer having a primaryand a balanced secondary, said primary being connected in said filternetwork so as to control the transmission characteristics thereof inaccordance with the primary impedance, a pair of space discharge devicesconnected to the balanced secondary and adapted to variably load thesame for altering the effective primary impedance of said transformer, acontrol channel responsive to the high frequency components of thesignal, means in said control channel to produce a control voltageproportional to the strength of said components and means applying saidcontrol voltage to said space discharge devices so as to change theimpedance thereof inversely as the strength of said components whereby abalanced load is applied to said secondary which varies directly as thestrength of said components.

12. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a transformer having a primaryconnected in said filter network and having a balanced secondary, meansvariably loading said secondary in accordance with the characteristicsof the impressed signal for altering the effective primary impedance,the primary impedance. when the secondary is unloaded, being such as todiscriminate against the higher frequency components of the signal, thediscrimination be-,

ing progressively reduced as the secondary is loaded and, underconditions of maximum load, discriminating somewhat in record.

' 13. In a system for recording or reproducingY sound, a transmissionchannel having-a lfilter network therein, a variable impedance in saidnetwork comprising the primary ofa transformer, means to variably loadand means responsive to the signal energy-for controlling said load sothat signals in'the background noise frequency range are only trans-'network therein, a transformer having a primary connected in said filternetwork to control, by its impedance, the transmission characteristicsthereof, a pair of variable impedance devices connected in balancedrelation to said secondary for applying a balanced load theretodependent upon their impedance, and means varying the impedance of saiddevices in accordance with the components of the signal whichare'normally suppressed by said filter.

15. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein including a shunt choke coil,said choke coil constituting a primary of a transformer, means applyinga load to the secondary of said transformer and means responsive to thesignal energy for automatically varying said load so as to change thecharacteristics of said filter in accordance with the characteristics ofsaid signal energy.

16. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein including a shunt choke coil,said choke coil constituting a primary of a transfavor of the lowerfrequencies so as to compensate for the under cutting of the lowerfrequencies on the sound, v st {nae-is aD the secondary ofsaidtransformer without changing the fiux density former, a variableimpedance device connected to variably load the secondary of saidtransformer and means responsive to the signal energy for automaticallyvarying the impedance of said device so as to change the load on saidtransformer in accordance with the characteristics of the signal energy.

17. In a system for recording or reproducing sound, a transmissionchannel having a filter network therein, a pair of transformers havingprimary windings connected respectively in shunt and in series in saidfilter network, means applying a variable load to the secondary windingsfor varying the effective primary impedances, and

cordance with the primary impedances, a pair of.

space discharge devices connected to each balanced secondary and adaptedto variably load the same for valteringthe effective primaryimpedanceoffth "transformers, a control channel responsivejto; ertaincomponents of the signal,

me s aid "ii -"trol .vO t-agefoliortion'al-tol'the strength of saidcomponents and: mfeans japplying said control :voltage to said- .spac e'discharge devices so'as to ntrolchannel to produce a con-.

h'a-rige-f the impedance; thereof inversely as the reng'th'of-saidicomponents whereby a balanced pliedfto' the secondaries whichvaries directly 'thej streng-thiof,saidcomponents.

'- 9 'In acsystem for recording or reproducing sound, havinga"-transmissio'nchannel including ;-a-varia-blelimpedance deviceconstituting the sec- 'o'nda'ry. of: a transformer, the method ofvarying .-'the' iinpedance of said device which comprises appilyinggavariable balanced load to the second'aryawhich-fvariesinproportion tothe characteristicsof the. signal, and causing the changes in thesecondary load to alter the effective pri- .ma'ry impedance of thetransformer in such manner that certain frequencies are suppressed bythe filter except when they are of a predetermined minimum strength.

20. In a system for recording or reproducing sound having a transmissionchannel including a filter network and a variable impedance device insaid filter network comprising the primary of the transformer, themethod of varying the transmission characteristics of said network whichcomprises variably loading the secondary of said transformer inaccordance with the signal while maintaining the flux density of saidtransformer substantially independent of said load.

21. In a system of the class described having an audio frequencytransmission channel including a variable filter, the method ofproducing t nal contrast which comprises utilizing the average volume ofthe high frequency tones applied to said channel for varying thefrequency transmission characteristics of said filter in such mannerthat the filter discriminates-in favor of low F (ill

